It makes sense, but I'm not sure how to implement it because example.com, sub.example.com, example2.com, and sub.example2.com are all subdomains (technically spoken).

Click to expand...

How about this:

Add Max. Host value

Remove the Hostname input field when creating a new Co-Domain

Add tab Hosts on and show the Hostname by Domain.

For new Host remove Domain input field

This way it doesn't matter if all Co-Domains are subdomain because you can only add Hosts to Domains already created.
ISPConfig has to check how many Domains, meaning domain.tld, the user can add as well as how many Hosts.

I fully Aggree with torusturtle's solutions - there are already to many websites were the dns is setup by "morons" were they omit the domain.tld resolution and only have www.domain.tld. I would in anycase like to see an tick at least allowing for an www sub-domain created automatically with each new domain made in the nameserver.